The plasma display technology now makes it possible to achieve flat colour panels of large size and with limited depth without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology.
One important quality criterion for a video picture is the Peak White Enhancement Factor (PWEP). The Peak White Enhancement Factor can be defined as the ratio between the peak white luminance level, to the luminance of a homogeneous white field/frame. CRT based displays have PWEF values of up to 6, but present Plasma Display Panels, (PDP), have PWEF values of about 4 only. Therefore, under this aspect the picture quality of PDPs is not the best and efforts must be taken to improve this situation.
First generations of PDPs were characterised by having a peak-white to maximum average luminance ratio (full-white image) of about 2. This has been improved during the last time to achieve a ratio of 4/5 mostly by using a dynamic control of sub-fields.
The Plasma display technology, being inherently digital, requires some other techniques than what is used for CRTs. CRTs use a so called ABL circuit (average beam-current limiter), which is implemented by analog means usually in the video controller, and which decreases video gain as a function of average luminance, usually measured over an RC stage.
A plasma display panel utilizes a matrix array of discharge cells, which could only be “on” or “off”. Also unlike a CRT or LCD in which gray levels are expressed by analog control of the light emission, a PDP controls the gray levels by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye time response.
Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more eye pulses and thus more “on” time. For this reason this kind of modulation is known as PWM, pulse width modulation. To establish a concept for this PWM, each frame will be decomposed in sub-periods called “sub-fields”. For producing the small light pulses, an electrical discharge will appear in a gas filled cell, called plasma and the produced UV radiation will excite a coloured phosphor, which emits the light.
In order to select which cell should be lighted, a first selected operation called “addressing” will create a charge in the cell to be lighted. Each plasma cell can be considered as a capacitor, which keeps the charge for a long time. Afterwards, a general operation called “sustaining” applied during the lighting period will accelerate the charges in the cell, produce further charges and excite some of the charges in the cell. Only in the cells addressed during the first selected operation, this excitation of charges takes place and UV radiation is generated when the excited charges go back to their neutral state. The UV radiation excites the phosphorous for light emission. The discharge of the cell is made in a very short period and some of the charges in the cell remain. With the next sustain pulse, this charge is utilized again for the generation of UV radiation and the next light pulse will be produced. During the whole sustain period of each specific sub-field, the cell will be lighted in small pulses. At the end, an erase operation will remove all the charges to prepare a new cycle.
More sustain pulses correspond to more peak luminance. More sustain pulses correspond also to a higher power that flows in the PDP. The PDP control can generate more or less sustain pulses as a function of average picture power, i.e., it switches between modes with different power levels depending on the picture content. The increase of the slope of the sustain pulses also correspond (non-linear) with more peak luminance.
The main objective is to optimize the contrast ratio without overstressing the power supply circuitry. In addition, the overall picture quality is linked to the number of sub-fields used for the grayscale rendition. The higher this number is, the better the picture quality is. Nevertheless, each sub-field introduces idle-time (death-time) for which no sustain can be made. When the number of sub-fields increases, the maximal number of available sustain decreases. For that reason, a strong compromise has to be made to optimize the picture luminance.
In a previous European Patent Application of the applicant, see WO 00/46782, a solution is described in which a control method generates more or less sustain pulses as a function of the average picture power, i.e., it switches between different modes with different power levels. This control method is characterized in that a set of power level modes is provided for sub-field coding, wherein to each power level mode a characteristic sub-field organisation belongs, the sub-field organisations being variable in respect to one or more of the following characteristics:                the number of sub-fields        the sub-field type        the sub-field positioning        the sub-field weight        the sub-field pre-scaling        a factor for the sub-field weights which is used to vary the amount of small pulses generated during each sub-field.        